Innovative DIY Sandwich Panels - Expedition Portal

Greetings, This is going to be a long post, so you may wish to make a cup of tea and get a note pad.

For many years now, the construction of sandwich panels or SIPs (Structural Insulated Panels), has been a bit of a mystery to many in the community. Notice, there is no such thing as a SIP Panel. You are saying Structural Insulated Panel Panel. It's just SIP. In the camper, trailer and RV world, insulation is generally not the focus of the build and isn't really considered at all by most builders. With cheap Chinese plastic windows, 1" x 1" or 2" x 2" dimensional lumber used in everything from new, entry-level trailers and campers to old FWC and Alaskan slide-ins, you sweat or freeze with the seasons. VIPs (Vacuum Insulated Panels) are the ONLY answer to the question, "How do you obtain a high R-value in a thin wall?" Alas, they are not accessible to most and I have only met one person who has attempted to make their own VIPs. If you stay plugged in, have a 13,000 BTU heater atop your 6' x 6' trailer or never venture into hot weather, insulation is less of a concern. However, if you take a moment to do some simple math, insulation pays dividends rather quickly. (see example below).

Foam, e.g. Styrofoam, a brand name, XPS, (a type of foam) GreenGuard, Kingspan, Dow, Pink Board and the like are generally available to John Q. Public. These products have been used to build campers, trailers, truck boxes, tiny boats and so on, generally with success. Moving into the commercial world, we have brands like Plascore, NidaCore, Diab/Divinycell, Gurit/CoreCell, Hexcell and others. These products are used to build go fast boats, aerospace bits and bobs, super-high end campers, race car parts and so on. These are not DIY friendly for the most part, generally due to cost and availability. High density Diab, for example comes in quite-odd sheet sizes, e.g. mm x mm or mm x 850mm. While this can be planned for in the design phase, these sheets do not match up well with standard U.S. sheets goods, which typically come in 4' x 8' or 5' x 10' or rolled goods such as Vetroresina.

When considering the use of pliable materials such as phenolic honeycomb Nomex or alloy-based honeycomb such as those produced by Hexcell and Plascore, this is a serious issue because the cells "stretch". You are not working with a rigid sheet of XPS having dimensions 4' wide and 8' long that you can lay onto a sheet of plywood, cut, glue, drill, etc. I have priced many of these materials, downloaded technical PDFs, spoken with sales people, application engineers, gone to trade shows, ordered sample materials and talked to builders and fabricators around the country, in an effort to figure out a relatively simple way to build a sandwich panel. Anyone who has followed the resin-infused carbon fiber camper being built on this forum or those who have played with composites, will quickly see what a pain in the ******** they are. Their properties are impressive. Working with them can be quite difficult and discouraging, however.

In my research, it seemed that it made the most sense to find the "best" core material and decide how far down the ladder a DIYer could go without sacrificing strength, longevity, durability, etc. Yes, I know people have built campers using old colouring books, wheat straw and left over mashed potatoes, but that is not what this thread is about. If you want to do that, knock yourself out; I don't. I have constructed one camper box using a modified version of a commercially-produced panel and I would not do that again. According to the current owner, the box has been bomb-proof. It is RIGIDLY MOUNTED, well-insulated and still going strong. Note by rigidly mounted, I mean to say there are no silly 47 point mounts, springs, doo-dads or the like. It is literally bolted to the frame of the truck, that's it. There are millions of truck bodies mounted this way the world over. This truck is no trailer queen either. It has been driven from Baja to Alaska, so the tech is sound. The down-side to that design is that it took forever to get the panels, they are no longer available, construction was quite labour intensive and the box was heavier than it needed to be.

To Be Continued... I see that there are two basic design strategies for building panels: (a) light and relatively strong, e.g. low-density foam of some kind and carbon or Kevlar skins or (b) something heavier and likely a bit stronger, but not necessarily, e.g. plywood, fiberglass, metal tubing or wood framing, etc.

In neither of these designs, is insulation (from heat or sound) generally considered. I go camping to get away from the hustle and bustle. I don't want to freeze my butt off or hear motorcycles riding through camp at midnight. So, there are two strategies to resolve this, in my opinion. Instead of trying to make one product do everything, i.e. strong, light, easy to build, good heat and sound insulation, readily available materials, and so on, you choose several products that excel at doing one or two things.

If you want a lightweight panel, you are going to be stuck using a low-density foam that isn't going to be super strong, and thin skins, nominally fiberglass, carbon fiber, Kevlar, Filon, aluminum, Vetroresina or the like. There is NOTHING wrong with this panel. It simply lacks ultimate strength, likely due to the core material chosen. A foam that weighs 1 to 5 pounds per cubic foot can only have so much strength. Is it fine for a small truck camper or DIY tow-behind camper trailer? Most certainly. Does it have the strength of a commercially-produced part from a defence contractor or an F1 team? Nope and that is OK. That strength isn't likely necessary.

The compromise comes in when the average DIY guy or girl looks at the cost of quality foams, resins, cloths, tools and labour - time is money - and says, "Heck no, man, I will just build my camper with plywood and TremClad." Again, there is nothing wrong with this, but that design lacks heat and sound insulation, ultimate strength, longevity, requires maintenance, etc. This is the path that most people choose and it is a good one for 90% of applications, so long as the wood is stained, sealed, charred, painted, oiled, etc., If it isn't, well, go look at an old Alaskan camper and you will see what I mean.

Exploring the marine, Formula 1 and aerospace worlds a few years ago, I came across a product that looks a lot like a bee honeycomb, made from a very thin aluminum foil.

Alloy Rigicell Aluminum Corrugated Honeycomb by HexCell with CR-PAA and CR III coatings. 1/8 – 2 – .-STD – 14.5 PPCF – PSI crush strength – to PSI compressive strength – 650 KSI compressive modulus – PSI shear – 260 KSI shear modulus (L) – PSI shear strength – 80 KSI shear modulus (W).

Compare this core material to Polypropylene honeycomb or a structural foam and you will see that it blows the doors off damn near everything. It is quite difficult to obtain and work with and is exhorbitantly expensive. "So, what comes close?", I asked myself. Well, aluminum is pretty light and strong, used in marine, transport and aircraft applications and is the basis of this core material for a reason. How could I construct a core out of aluminum without the specialised equipment required to make this honeycomb? Use a cheaper, lighter, simpler version of what is already on the market, is the answer. I got samples of Coosa BlueWater 15, 20 and 26, Celtec, Plascore, HDPE and several structural foams and wasn't really impressed with any of them once costs, lead times, shipping, sound damping, insulation and all other factours were considered.

Ultimate strength version, e.g. truck bodies, trailer decks, boat hulls, etc.: (not small, lightweight camper boxes)

Purchase a 2" x 2" x 0. - 0.125" wall square tube from your favourite metal supplier, along with a 0. to 0.125" thick sheet for the skin. Lay out all of the tubes, edge to edge - think roll up sushi mat, and tape (VHB), glue or weld them together. This is your core. You should have something that looks like the inside of corrugated cardboard or a DIY log raft, when you are done. Next lay the skin of your choice over this core material and tape, bond or weld it in place, repeat for the other side.

You now have an incredibly strong panel that is completely customisable by changing the width, height and wall thickness of the tubing used for the core, as well as the material type of the core and skin. An all-aluminum sandwich panel will have terrible acoustical and insulative properties, so you will need to make use of radiant barrier coatings, room-in-a-room, Sylomer, Aliphatic Polyurea or other products to resolve these problems. A foam cored panel will have much better insulation, for example, but there is nothing that you can do to it to make it "strong". You can insulate and damp a strong panel by using other products designed to accomplish those tasks.

Another option is to make use of a plastic tubing, such as this one, for the core material.

2" OD x 0.078" Wall PVC Square Hollow Tubing | U.S. Plastic Corp.

This PVC hollow square tubing is priced per foot and sold in 10' lengths only. 10' lengths are nominal. The color is dark gray.
This tubing would replace the alloy tubing in the example above and could be skinned with composite sheeting, such as Filon, Phenolic, Vetroresina or even a thin sheet of aluminum or sealed plywood. In a sandwich panel of any design, the skins carry tensile (30-42k for ) and compressive loads (10M PSI for ) and the core carries the shear load ( – PSI for 14.5 PPCF Hexcell). So, you want a thick core (taller I-Beam) with high shear strength, and skins with high tensile and compressive strength. Cheap sandwich panels often fail due to weak cores, placing stress on thin/cheap skins, that quickly buckle once they are no longer supported by a core.

Lightweight/cheap version: Get XPS foam from your local hardware store, along with your favourite sheet goods. Insert framing members (dimensional lumber, alloy square tubing, etc) to carry the shear load, as foam sucks at this job. Laminate the framed core, i.e. a picture frame or a tic-tac-toe grid made with tubing and the foam as the picture, plugs or panels within your chosen structure and skin combination. This panel will not have the very best strength-to-weight ratio, but it will be fairly light and will have better insulation than the alloy tubing based panel above. Alternatively, exterior framing can be attached to the panel or the finished camper box, in the form of aluminum, steel or fiberglass angle iron to stiffen the structure and reduce shearing/racking loads on the panel and its quite weak, by comparison, foam core.

The strongest core material that I can find on the market has a shear strength of about 3,000 PSI. It is absurdly expensive, has a long lead time, and is quite difficult to work with, according to the manufacturer. An off-the-shelf tube that anyone can buy, has a shear strength 10 TIMES this amount and is quite easy to work with, by way of comparison. Why insulation is important in your sandwich panel or why you should add it to a purely structural panel that has good strength, but poor insulation.

To convert the U Factor from BTUs to Watts, simply multiply the value in BTUs by 5.678. For example, if a window has a U Factor of 0.21 BTU/h·ft2·F (R-4.76), then 0.21 x 5.678 = 1.19 (W/m2K).

If Thermal Conductivity is given in BTU-in/ft2-hr-°F, divide the thickness of the material in inches by the Conductivity to get R-value.

I want

K-Factor

C-Factor
R-Factor
I Have

K Factor

C=K-factor/in. of thickness

R= in. of thickness/K-factor

C Factor

K=C-factor in. of thickness
R=1/C-factor
R Factor

K=in. of thickness / R-factor

C-1/R-factor

None of
the Above
K=BTU-in / hr - ft² - °F


C=BTU/(hr · ft · °F)
R=h · ft² · °F/BTU
The K Factor of Insulation
The K factor of insulation represents the material's thermal conductivity or ability to conduct heat. Usually, insulation materials have a K Factor of less than one. The lower the K factor, the better the insulation. The textbook definition of the K factor is “The time rate of steady heat flow through a unit area of homogeneous material induced by a unit temperature gradient in a direction perpendicular to that unit area.” Simplified, the K factor is the measure of heat that passes through one square foot of material that is one inch thick in one hour. Thus, 1” XPS (R-5) has a K-value of 0.2, or 1/5.

How Do I Calculate the K Factor of Insulation?
If R factor is unknown, the formula to calculate the K factor of insulation is:

K factor = BTU-in / hr - ft2 - °F or British Thermal Unit-Inch Per Square Foot Per Hour Per Degree F.

If R factor is known, this easier formula can be used to calculate the K factor:

K factor = inches of thickness / R Factor (total) : 2” /R-10 = 1” / R-5.

How is the K Factor of Insulation Reported?
K factors are reported at one or many mean temperatures. The mean temperature is the average of the sum of the hottest and coldest surface temperatures which the insulation material is exposed to. The testing apparatus that determines the K factor of an insulation material places a sample of the material between two plates, hot & cold, and the average of the surface temperatures of those two plates equals the mean temperature.

The C Factor of Insulation
The C factor stands for Thermal Conductance Factor. The C factor, like the K factor, is a rate of heat transfer through a material. The lower the C factor, the better the insulating properties of the material. It is the quantity of heat that passes through a square foot of insulation material. One of the main differences between the K factor and C factor, is that, generally, the thickness of an insulation material will not affect its K factor. In the inch-pound units used in the United States, the units for C-factor are BTUs per hour per square foot per degree F of temperature difference. The words are fairly similar to those in the definition for thermal conductivity. What is missing is the inch units in the numerator because the C-value for 2” of XPS is half the value that it is for 1” of XPS. In other words, the thicker the insulation, the lower its C-value, as the thicker insulation conducts less heat through it.

How Do I Calculate the C Factor of Insulation?
If the K factor is unknown, the formula to calculate the C factor of insulation is:

BTU/(hr·ft⋅°F) or BTUs/hour per square foot per degree F of temperature difference

If the K factor is known, this easier formula can be used:
C factor = K factor / inches of thickness of insulation (total or per piece), e.g. inverted R-10 = 0.1

What is the R-Factor or R-Value of Insulation?
The R factor pulls together all of the information of the other factors and makes it easy to judge the effectiveness of insulating material. The R factor is the most popular indicator of a material's insulator properties and is generally listed on an insulation material's label. The R factor is the opposite of C-factor, in the it is a measurement of the resistance of heat transfer through a material. The textbook definition for R Factor is: the quantity determined by the temperature difference, at steady state, between two defined surfaces of a material or construction that induces a unit heat flow through a unit area. To simplify, the R factor is a variable value that measures the ability of a material to block heat rather than radiate it or conduct it. The variable is the C factor, which is dependent upon the thickness of the material. R-value is the resistance to the flow of heat energy through the insulation.

How Do I Calculate the R Factor of Insulation?
There are a few formulas to calculate the R factor of insulation, depending on if your K factor and C factor are known. If they are unknown, you can use this formula:

h·ft²·°F/BTU or degrees F times square feet of area times hours of time per units of heat (flow)

If your K factor and C factor are known, you can use these formulas which may be easier to use:

R-factor = 1 / C-factor OR R-factor = thickness in inches / K-factor

R-1 per inch means that 1 BTU/hr/1*F/1”/PSF of material is resisted. A 2” thick piece of the same material with an R-6 / inch flows 1/12th of a BTU/hr per degree F* /1”/PSF of material.

This is a great site that explains the differences between C, K and R-values: (source of info above)
https://blog.thermaxxjackets.com/insulation-ratings-r-factor-k-factor-c-factor

Let's distill that down to a real world example that we can all relate to.

You have just purchased your first camper, trailer, RV, tiny house or what have you. The walls were built using 1" x 2" lumber and some fiberglass skins. The R-value (resistance to heat transfer) quoted by the manufacturer is R-5.

For this example, your camper is 7' tall, 7' wide and 12' long, a rectangular box. Not counting doors and windows, which are notoriously leaky and poorly insulated, you have 434 square feet of 'panel'. Assuming even heat loss, again, for the exercise, and R-5 walls, over your camper box, you need BTU/hr just to maintain 70*F if it is 20*F outside. This does not "warm up" the box, this is just to "put back" the heat that is lost every hour, through the walls. In the real world, heat lost through vents, windows, seals, fans, doors and hatches, is probably several times this amount. Now, you see why OEMs put 13,000 BTU/hr heaters in 6' x 6' truck campers. 1 kW = 1.34 HP = BTU/hr. If you were running a generator, you would need a 3-4 HP generator running at full load/throttle just to maintain temperature in your camper box, in this scenario.

If, instead, you have a well-sealed 4" thick wall that is R-30 or a VIP-based wall that is R-60, you now only need 723 BTU/hr or 361 BTU/hr, respectively. The latter is equivalent to running a 100 Watt heater to heat your camper and the former requires twice that amount.

As a side note, most windows have an R-value of 1 or 2, the same as wood. The best windows on the planet, triple-pane, Krypton-filled and low-e coated, have an R-value of about 10 or 11 and are absurdly expensive. Would you build a house with an R-2 or even an R-5 wall? That would not pass code anywhere in the country, if I had to guess.